Image tube having a gating and focusing electrode



Oct. 21, 1969 R G. STQUDENHElMER ET AL 3,474,275

IMAGE TUBE HAVING A GTING AND FOCUSING ELECTRODE Filed Sept. 26, 1966 United States Patent O 3,474,275 IMAGE TUBE HAVING A GATING AND FOCUSING ELECTRODE Richard G. Stoudenheimer and Lawrence A. Ezard, Lancaster, Pa., assignors to RCA Corporation, a corporation of Delaware Filed Sept. 26, 1966, Ser. No. 581,874 Int. Cl. H011' 31/50 U.S. Cl. 313-65 4 Claims ABSTRACT OF THE DISCLOSURE A light-shutter image tube having an electrode relatively close to a spherically curved photocathode. The electrode serves the two functions of gating and focusing. In order to produce a flat electron optical image at a flat phosphor screen, the radius of curvature of the photocathode has a length that is from 0.7 to 0.9 times the distance from the photocathode to the axial crossover point of trajectories of electrons emitted by the photocathode.

Our invention relates to image tubes and particularly to improvements in light-shutter tubes for use in high speed photography.

One type of light-shutter image tube includes a gating electrode adjacent to a photocathode. The gating electrode has an aperture crossed by a plurality of equally spaced fine metal wires for passing or blocking the electron image produced by the photocathode. A focusing electrode spaced from the end of the gating electrode remote from the photocathode, serves to focus the electrons passed by the gating electrode. An anode electrode is provided t direct the focused electrons to a phosphor viewing screen.

Several disadvantages characterize prior image tubes of this type. Electronic shutter tubes using grid wires are characterized by relatively low resolution and image break-up near the grid wires. Also, in tubes of this type, the resolution away from the tube axis is much lower than `the resolution on the tube axis.

A further disadvantage of prior light-shutter image tubes, as well as other electrostatically focused image tubes, is that image aberrations results from failure `to produce a at electron optical image at a flat phosphor screen. Such aberrations result mainly from an improper curvature of the photocathode and may be manifested as astigmatism and undesired field curvature. The curvature of the photocathode is usually limited by the image curvature possible with an optical lens which forms a light image on the photocathode. The electron optical aberrations referred to are reduced when the radius of curvature of the photocathode is reduced. However, if the radius of curvature of the photocathode is reduced, it becomes increasingly difficult to design a high quality light optical system which will produce an image lield curved to precisely the same radius as the photocathode of the image tube.

Accordingly, it is an object of our invention to provide an improved image tube.

A further object is to provide a light-shutter tube of improved resolution.

Another object is to provide an image tube characterized by reduced image aberrations.

The foregoing and other objects are achieved in accordance with the subject matter disclosed herein by providing a tubular electrode that serves the two functions of gating and focusing. The tubular electrode is provided With a free opening coextensive with the cathode, thereby avoiding the loss in resolution caused by wires across the opening. We have found that one electrode can effectively perform the two functions of gating and focusing since lCe for gating, does not interfere with the subsequent focusing operation of the electrode. When it is desired that the electrode pass electrons, a relatively low positive voltage thereon satisfactory for focusing the emission from the photocathode, may be used.

The aforementioned feasibility of using only one electrode for both gating and focusing is of advantage in connection with our discovery of a desired spacing between the photocathode and the electron cross-over point of the electron optical system, for improved optical results, We have found that such space between the photocathode and the electron cross-over point should be slightly greater than the radius of curvature of the photocathode. When the radius of curvature of the photocathode is from 0.7 to 0.9 times the distance from the photocathode to the cross-over point, best results are obtained. For best operation of the tube we have found it desirable to position the anode of the tube so that the end of the anode adjacent to the photocathode is disposed substantially in the plane of the electron cross-over point. Such disposition would not be feasible because of space limitations if a plurality of electrodes were required between the photocathode and the anode.

In the drawings, to which reference is now made for an exemplary embodiment of the invention, there is shown in the single figure thereof an image tube in which our teachings are incorporated.

The image tube shown comprises a generally cylindrical envelope 10 having at one end thereof a spherically curved glass faceplate 12 coated on the inner concave surface thereof with a layer 14 of suitable material that emits electrons in response to light directed to the outer surface of the faceplate 12. Suitable materials for forming the layer 14 are: cesium, potassium, sodium, an antimony, constituting a multialkali photocathode 14 of S-20 response. The layer 14 constitutes a photocathode 14 extending across substantially the entire inner surface of the faceplate 12.

At the other end of the envelope 10 is a viewing screen 16 adapted to respond in a suitable visible display to electrons impinging thereon from the photocathode 14. The viewing screen 16 :comprises a suitable substrate 18, made of glass for example, and having a layer 20 of a suitable phosphor such as silver-activated zinc cadmiumsulfide (P20) on the inner surface thereof. The phosphor layer 20 is substantially coextensive with the photocathode 14.

Internally of the envelope 10 and between the photocathode 14 and phosphor screen 20 is disposed a metal grid electrode 22 having a cylindrical portion 24 fixed to a metal ring 26 sealed through glass portions 28 and 30 of the tube envelope 10. A cylindrical metal cylinder 32 is telescoped into the cylindrical portion 24 of the grid electrode 22 and is suitably fixed therein. The end of metal member 32 adjacent to the photocathode 14 is inturned at 34 to provide an opening 36 substantially coextensive with the photocathode 14. The electrode 22 also includes a frusto-conical portion 38 fixed to the glass portion 30 of the tube envelope 10 and physically joined to the electrode portion 24 as by a braze 40. If desired, in the interests of easy assembly, the electrode portion 38 may be spaced from the grid portion 24 without appreciably disturbing the arrangement shpwn, but in this event a suitable lead interconnecting grid portions 38 and 24 is desirable, since in operation the two portions of the electrode 22 should preferably have the same potential thereon. The electrode 22 with portion 38 thereof constitute a single electrode.

A tubular metal anode 42 is supported on a metal ring 44 fixed to a metal portion 46 of the tube envelope 10 and is disposed between the electrode 22 and the phosphor screen 20. The anode 42 includes a frusto-conical portion 48 extending into the frusto-conical portion 38 of the electrode 22. While the anode 42 could be cylindrical throughout its length, the frusto-conical shape of portion 48 is of advantage in forming the electrostatic field for the electron optical lens system used in the tube.

The envelope is evacuated through a metal exhaust tubulation 54 disposed adjacent to the photocathode 14. The exhaust tubulation 54 is shown sealed by a pressure weld.

The structure shown embodies features that contribute advantageously to a gating function required in light- `shutter operation. In this type of operation the electron image produced by the photocathode 14 is successively passed and blocked by the electrode 22. To this end, the electrode 22 is connected to a suitable pulsing circuit 52, which is of a type that is available in the art, to successively impress on the electrode 22 voltages of from about +200 to +350 volts for passing and focusing the electron image from the photocathode 14, and voltages from about -1100 to about 1300 volts for blocking the electron image. With the type of cathode structure shown, electronic shutter action involving periods as short as five micro-seconds is feasible Without adverse effects on the image resolution. Shorter periods of shutter action are feasible by interposing a conducting layer (not shown) between the photocathode 14 and the transparent insulating substrate 12. In the example shown, the photocathode 14 extends into contact with metal envelope portion 53 which is usually kept at ground potential during operation. The conducting coating, if used, would also be in electrical contact with envelope portion 53.

The achievement of desired electronic shutter action with a single grid electrode 22 serving the two successive functions of stopping and passing the electron image from the photocathode 14, is due appreciably to a relatively close spacing between the electrode 22 and the plane defined by the periphery of the photocathode 14. Such relatively close spacing in the tube structure shown is about 0.050 inch. This spacing gives the best results. Satisfactory results are obtained if this spacing is varied from about 0.04 inch to about 0.10 inch. Also contributing to the feasibilty of using a low voltage on the electrode 22 is the coextensive relation of electrode aperture 36 with the photocathode 14, and the location of the upper end 56 of frusto-conical portion 48 of the anode 42 below the plane of the upper opening 58 in the frusto-conical portion 38 of electrode 22, .as viewed in the drawing. This disposition of the anode portion 48 is significant since it prevents the relatively high positive voltage, i.e., about +15,000 volts impressed on the anode 42 during operation from adversely affecting the blocking of the electron image during a period when the electrode 22 is impressed with a blocking voltage.

The disposition of the frusto-conical portion 48 of the anode 42 in partly telescoped relation with the frustoconical portion 38 of the electrode 22 is of advantage in securing an axially compact tube and is of important utility in preserving the electron image from effects that produce image aberrations such as astigmatism and field curvature of the electron image adjacent to the phosphor screen 20.

We have discovered a desirable relationship between the radius of curvature 59 of the photocathode 14 and the axial distance 60 from the photocathode 14 to the axis crossover point 62 of electron trajectories 64 from the cathode, for avoidance of image aberrations. We have found that this relationship is satisfied when the radius of curvature 59 of the photocathode 14 is equal to from about 0.7 to about 0.9 times the axial distance from the central or axial portion of the photocathode 14 to the crossover point 62 of electron trajectories 64. Within this range, best results are obtained when the radius of curvature 59 of the photocathode 14 is about 0.8 times the axial distance referred to. The axial distance from the photocathode 14 to the axis crossover point 62 of electron trajectories 64 is therefore slightly larger than the radius of curvature 59 of the photocathode. 14.

In the structure shown, the plane of the end or opening 56 of the frusto-conical anode portion 48 is very close to, i.e. substantially includes, the axis crossover point of electron trajectories from the photocathode 14. In other tubes having coaxial electrodes of different construction than shown in the drawing, an end of the grid electrode remote from the phosphor screen may be axially spaced appreciably from the axis crossover point of electron trajectories. However, in such tube the advantage of avoiding image aberrations may also be realized by adjusting the axial spacing between the photocathode and the axis crossover point of the electron trajectories so that this spacing is larger than the radius of curvature of the photocathode within the range indicated before herein. It is thus clear that the advantages of our invention in respect of freedom from image aberrations, can be realized in tubes having either conical or cylindrical grids positioned independently of the axis crossover point of electron trajectories.

Typical approximate dimensions characterizing the tube shown are:

Inches Overall length of tube enevlope 10 4.825 Diameter of tube envelope 10 3.0 Axial length of electrode portion 32 1.0

Axial length of electrode portion 38 0.83

Typical approximate operating voltages of a phototube herein discussed, are:

Voltage Anode 42 voltage +15,000 Operating voltage of grid 22 +200 to +350 Cut-off voltage of grid 22 -1100 to +1300 Our improved electronically shuttered image device is characterized by many advantages over devices of this type in the prior art. Our improved device has a resolution that is greatly superior to prior devices of this type. In such prior devices a limited area of active photocathode surface was required with ya result that maximum resolution capabilities were from about 22 to 30 line pairs per millimeter. Our improved device permitting the use of a photocathode substantially coextensive with the faceplate of the device, is characterized by a uniform resolution of from about 55 to 70 line pairs per millimeter over a 40 millimeter diameter photocathode. `Coupled with this substantial increase in resolution, 'the improved electronoptical system of our device reduces to insignificant levels any image aberrations present in prior art image devices. Among the more important of such image aberrations substantially reduced by our system are those caused by astigmatism and a curvature of the electrostatic image field.

Our improved electron optical system contributes to important structural advantages of an electronically shuttered image device. Since the photocathode may be fully coextensive with an end faceplate of the device, our device may have a greatly reduced diameter such as about 3 inches. Furthermore, the use of one electrode to serve the two functions of gating and focusing, makes it feasible to reduce the overall device length to a value as small as 4.825 inches, for example. These reductions in device dimensions do not adversely affect utilization of vour improved device in applications where devices of appreciably larger dimensions were required heretofore.

It is apparent, therefore, that We have provided an image device that not only is superior to prior art devices in respect of the electron-optical system incorporated therein, but also in respect of -appreciably smaller dimensions that may characterize its structure.

We claim:

1. An image device comprising:

(a) an electron emitting spherically curved photocathode having an axis and adapted to release an electron image in response to the impression of a light image thereon,

(b) a phosphor screen adapted to display a light image in response to the impression of said electron image thereon in its movement along a path between said photocathode and screen, the electrons of said electron image having trajectories crossing over at a point along said axis,

(c) a tubular anode electrode in said path for accelerating said electron image to said screen, and

(d) a single relectrode only, between said anode and photocathode, said electrode being free from electrical contact with said photocathode and surrounding a portion of said path and adapted sequentially to stop travel of said electron image to said screen and to pass and focus said electron image,

(e) the radius of curvature of said photocathode being from about 0.7 to about 0.9 times the distance fro-m said photocathode to said point at which said electron trajectories cross over along said axis.

2. A light-shutter image device comprising:

(a) an electron emitting photocathode having a spherically curved surface,

(b) a phosphor screen spaced axially from said photocathode,

(c) two electrodes only, between said photocathode and said screen,

(l) one of said electrodes comprising a metallic structure combining the function of gating on and off an electron image from said photocathode and focusing said electron when the same is gated on,7

(2) the other of said electrodes comprising an anode for directing to said screen the focused electron image when gated on,

(d) said anode having `an open end remote from said screen and defining a plane substantially including the `axial cross-over point of electron trajectories from said photocathode,

(e) the radius of curvature of said photocathode being from 0.7 to 0.9 times the distance from said photocathode to said plane.

3. A light-shutter image device comprising:

(a) Va spherically curved photocathode,

(b) -a planar phosphor screen spaced axially from said photocathode, and

(c) an electrode closely adjacent to said photocathode for sequentially passing and blocking the electron image from said photocathode and for focusing said image when said image is passed,

(d) said photocathode having a radius of curvature substantially equal to from about 0.7 to `about 0.9 times the axial distance from the photocathode to the crossover point of trajectories of said electron emission,

(e) whereby said phosphor screen is adapted to provide a visible display characterized by increased resolution and substantially free from astigmatism `and curvature of the electrostatic field lformed by said emission adjacent to said screen.

4. In an electronically shuttered image device:

(a) aphotocathode,

(b) a phosphor screen positioned to receive electron emission from said photocathode and to produce a visible display in response thereto, and

(c) an electrode between said photocathode and screen and closely adjacent to said photocathode,

(d) said electrode being responsive to energizing means connected thereto, for sequentially passing and blocking electron emission from said photocathode and for focusing said image when said image is passed,

(e) said photocathode having -a spherical curvature and adapted to produce an emission of electrons having trajectories crossing over at a point within said device,

(f) the radius of said spherical curvature of said photocathode being substantially equal to from about 0.7 to about 0.9 times the distance from the center of said photocathode to said electron trajectory crossover point.

References Cited UNITED STATES PATENTS 3,303,345 2/1967 Wulms 313-65 X 2,421,182 5/1947 Bayne 250-213 2,172,728 9/1939 Brche 313-65 2,235,831 3/1941 Coeterier 313-65 2,757,293 7/1956 Teves et al 315-10 X 3,280,356 10/1966 Stoudenheirner et al. 313-65 JAMES W. LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner U.S. Cl. X.R. 

